1. Field of the Invention
The present invention relates to a sensor failure detection system for an air-to-fuel ratio control system of a type having air-to-fuel ratio sensors in front of, or before, and behind, or after, a catalytic converter.
2. Description of Related Art
Automobile internal combustion engines are typically equipped with a catalytic converter for emission control or exhaust gas purification. In order for the ability of the catalytic converter to be maximized, an exhaust sensor of an air-to-fuel ratio control system, such as an oxygen (O.sub.2) sensor disposed before the catalytic converter, detects the oxygen content of exhaust gas. The air-to-fuel ratio control system determines the proper air-to-fuel ratio and then constantly monitors engine exhaust to verify the accuracy of the mixture setting. Specifically, whenever the exhaust sensor determines that the oxygen content is improper or "off", the air-to-fuel ratio control system corrects itself to bring the oxygen back to proper levels or predetermined threshold values and tries to maintain a stoichiometric air-fuel mixture, or ideally combustible air-to-fuel ratio, in a feedback control range which is predetermined according to engine speeds and engine loads.
For the purpose of providing a brief description of closed loop air-to-fuel ratio controls or feedback air-to-fuel ratio controls in such single exhaust sensor types of air-to-fuel ratio control systems, reference is made to FIGS. 7-9.
As shown by a time chart (A) of an output voltage Vc of the exhaust sensor in FIG. 7, the air-to-fuel ratio control system judges an air-fuel mixture to be lean when the output voltage Vc varies by passing through a judging level Va, predetermined at a median of latitude in output change, from a value in a lean region to a value in a rich region. The air-to-fuel mixture is judged to be rich when the output voltage Vc varies by passing through the judging level Va. According to a result of the judgement, the air-to-fuel ratio control system changes a feedback control factor CFB, as indicated by a time chart (B) in FIG. 7, at a time at which the output voltage Vc reaches and passes the judging level Va. In this instance, because a response time of the exhaust sensor is different between when the output voltage Vc varies from the lean region to the rich region and when the output voltage Vc varies from the rich region to the lean region, the air-to-fuel ratio control system varies the feedback control factor CFB with a delay time (TRL or TLR) from the judging time. Specifically, the system increases the feedback control factor CFB so as to enrich an air-fuel mixture after the delay time TRL from the time of lean judgement. Similarly, the system increases the feedback control factor CFB so that an air-fuel mixture gets leaner after the delay time TLR from the time of rich judgement. In the time chart, a leap or skip (PRL or PLR) represents what is called a P-value which is a proportional term of the feedback control factor CFB.
This type of exhaust sensor experiences (1) variations in characteristics and (2) deterioration in sensing ability due to aging. Such variations and deterioration have an adverse effect on performance of catalytic converters and, consequently, provide a decline in emission control.
The exhaust gas purifying efficiency of a catalytic converter is expressed by oxygen consumption. Therefore, the catalytic converter has a property such that deviations from the optimum purifying efficiency, which is obtained at an ideally combustible air-to-fuel ratio (expressed by .lambda.=1), can be found on the basis of the oxygen content of exhaust gas downstream from the catalytic converter. By utilizing this property and providing an exhaust sensor as a monitor, downstream from the catalytic converter, it is possible to use the air-to-fuel ratio control system to compensate for variations in and deterioration of characteristics of exhaust sensors and to detect failure of the exhaust sensor and/or characteristic deterioration of the catalytic converter. In other words, the detection of failure of the exhaust sensor and/or characteristic deterioration of the catalytic converter is made by controlling an air-to-fuel ratio on the basis of a feedback control factor. The feedback control factor is determined according to an output of the exhaust sensor (which is hereafter referred to as the air-to-fuel ratio control sensor) upstream from the catalytic converter and corrected according to an output of the exhaust sensor (which is hereafter referred to as the air-to-fuel ratio monitor sensor) downstream from the catalytic converter. This air-to-fuel ratio control system is called a double sensor type system.
One air-to-fuel ratio control system of this double sensor type provides what is called leap or P-value feedback control. In such a control, a leap value PRL or PLR is controlled on the basis of an output of the air-to-fuel ratio monitor sensor. Such an air-to-fuel ratio control system of this P-value feedback control type is described in, for instance, Japanese Unexamined Patent Publication No. 62-147034. The approach used is to execute the P-value feedback control exclusively when predetermined conditions are satisfied, in a predetermined range of engine speeds and engine loads, for air-to-fuel ratios for feedback control in an interval after a predetermined time from engine starting.
Time charts (A) and (B), depicted in FIG. 8, show an output voltage Vc of a control sensor upstream from a catalytic converter and an output voltage Vm of a monitor sensor upstream from the catalytic converter, respectively. The output voltage Vm, which is different in phase from the output voltage Vc and has a frequency smaller than the output voltage Vc, changes above and below the judging level Vb taken as the center of change. Time charts (C) and (D), depicted in FIG. 8, show correction coefficients CGPfRL and CGPfLR for leaps PRL and PLR, respectively. When the output voltage Vm of the monitor sensor is above a judging level Vb, it is judged that an air-to-fuel ratio is such that the air and fuel mixture is rich. The leap correction coefficient CGPfRL is then decreased. On the other hand, the leap correction coefficient CGPfLR is increased. As a result, when the output voltage Vm of the monitor sensor is above the judging level Vb, a feedback control factor CFB, indicated by a time chart (E) in FIG. 8, is gradually changed toward a larger negative or minus value. On the other hand, when the output voltage Vm of the monitor sensor is below the judging level Vb, it is judged that an air-to-fuel ratio is such that the air fuel mixture is lean. The leap correction coefficient CGPfRL is increased. The leap correction coefficient CGPfLR is decreased. As a result, when the output voltage Vm of the monitor sensor is below the judging level Vb, the feedback control factor CFB is gradually changed toward a larger positive or plus value.
In the known air-to-fuel ratio control system of the double sensor type mentioned above, when each of these correction coefficients CGPfRL and CGPfLR or, otherwise, the average value of these correction coefficients CGPfRL and CGPfLR, exceeds a predetermined value, it is determined that the control sensor upstream from the catalytic converter is out of order or abnormal.
If something should go wrong with the monitor sensor, the air-to-fuel ratio control will inaccurately detect functional deterioration of the control sensor and will experience deteriorated emission control, functional deterioration of the catalytic converter and the like.